WASHINGTON (AP)  Osama bin Laden may be slipping back and forth from Pakistan to Afghanistan. Or the U.S. might not have a clue, more than eight years after the al-Qaeda leader masterminded the terrorist attacks on America.

Given a chance Sunday to clear away some of the mystery surrounding the whereabouts of the world's most wanted terrorist, Obama administration officials seemed to add to it with what appeared to be conflicting assessments.

President Barack Obama's national security adviser, James Jones, said bin Laden, believed hiding mainly in a rugged area of western Pakistan, may be periodically slipping back into Afghanistan. But Obama's Pentagon chief, Robert Gates, said the U.S. has lacked good intelligence on bin Laden for a long time — "I think it has been years" — and did not confirm that he'd slipped into Afghanistan.

The failed hunt for bin Laden has been one of the signature frustrations of the global war on terrorism that former President George W. Bush launched after the Sept. 11 attacks. The main explanation given by both the Bush and Obama administrations for not getting bin Laden is that they simply don't know where he is.

Jones, a retired Marine general, stressed the urgency of targeting bin Laden, and spoke of a renewed campaign to capture or kill him. Bin Laden had been sheltered in Afghanistan by Taliban allies while plotting the Sept. 11 attacks. When U.S. forces ousted the Taliban in late 2001, bin Laden fled into Pakistan from his mountain redoubt.

Asked on CNN's State of the Union whether the administration has reliable intelligence on bin Laden's whereabouts, Jones replied, "The best estimate is that he is somewhere in North Waziristan, sometimes on the Pakistani side of the border, sometimes on the Afghan side of the border." He did not comment on the intelligence behind that estimate, nor did he cite a time period or describe more specifically bin Laden's apparent border crossings.

Gates told ABC's This Week that "we don't know for a fact where Osama bin Laden is," although he agreed that his likely location is North Waziristan.

That's part of the loosely governed Federally Administered Tribal Areas of northwest Pakistan where the border with Afghanistan is largely unrecognized and unmarked. There is little Pakistani government or military control in this remote region, and militants affiliated with al-Qaeda can move freely across the frontier into Afghanistan.

The U.S. has targeted North Waziristan and other areas on the Pakistan side of the border with drone-launched missile strikes, killing substantial numbers of militants as well as Pakistani civilians. The Pakistani army has undertaken an offensive against Taliban militants in South Waziristan but it has not expanded the effort into North Waziristan.

Obama administration officials have often asserted, as did the Bush administration, that they believe bin Laden is being sheltered on the Pakistani side of the border, along with other senior al-Qaeda leaders. But Jones broke new ground by saying publicly that the al-Qaeda chief may have slipped back into Afghanistan.

Sen. John McCain, R-Ariz., made a somewhat similar, if less specific, remark Sunday about bin Laden's movements. He told NBC's Meet the Press that knowledgeable people have told him that bin Laden "moves back and forth."

McCain did not elaborate, except to say that although bin Laden is not currently able to establish bases for training and equipping terrorists who would attack the United States, "I think it's important to get him."

Two Afghan provinces in the country's northeast held particular attraction for bin Laden in the 1990s: Kunar and Nuristan. The towering mountains there hid bin Laden training camps that date back to the Soviet occupation of Afghanistan in the 1980s. A longtime bin Laden ally, Gulbuddin Hekmatyar, holds sway in the area. U.S. troops have targeted Hekmatyar's security chief, Kashmir Khan, in Kunar.

During his years in Afghanistan as a guest of the Taliban, bin Laden operated mainly in the southern region around Kandahar.

Gates said he does not blame a lack of Pakistani cooperation for the absence of intelligence on bin Laden.

"No, I think it's because if, as we suspect, he is in North Waziristan, it is an area that the Pakistani government has not had a presence in, in quite some time," Gates said, adding that although the Pakistani government has its own priorities, any pressure it brings on the Taliban is helpful because it is in league with al-Qaeda.

During a visit to Pakistan in late October, Secretary of State Hillary Rodham Clinton caused a stir by chiding Pakistani officials for failing to press the hunt for al-Qaeda inside their borders. She said she found it "hard to believe" that no one in Islamabad knows where the al-Qaeda leaders are hiding and couldn't get them "if they really wanted to."

Gates said he could not confirm recent news reports that bin Laden had been seen in Afghanistan earlier this year. BBC News reported last week that a Taliban detainee in Pakistan claimed to have met in January or February with an unidentified associate who said he had seen bin Laden just days earlier in Afghanistan, possibly in Ghazni province.

A recent Senate report said bin Laden was unquestionably within reach of U.S. troops in the mountains of Tora Bora in eastern Afghanistan only three months after the attacks of Sept. 11, 2001, when American military leaders made the crucial decision not to pursue him with massive force.

The report asserted that bin Laden's escape at his most vulnerable in December 2001 laid the foundation for today's reinvigorated Afghan insurgency and inflamed the internal strife now endangering Pakistan. Staff members for the Senate Foreign Relations Committee's Democratic majority prepared the report at the request of the chairman, Sen. John Kerry of Massachusetts.

Tommy Franks, the retired Army general who ran the initial war effort in Afghanistan and chief of U.S. Central Command, wrote in his book, American Soldier, in 2004 that he was confident in late 2001 that al-Qaeda could not escape the Afghan forces leading the battle around Tora Bora, supported by heavy air strikes from American warplanes.

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